An electrically-variable transmission (EVT) splits mechanical power between an input shaft and an output shaft into a mechanical power path and an electrical power path by means of differential gearing. The mechanical power path may include clutches and additional gears. The electrical power path may employ two electrical power units, each of which may operate as a motor or as a generator. With an electric storage battery, the EVT can be incorporated into a propulsion system for a hybrid electric vehicle.
The hybrid vehicle or hybrid propulsion system uses an electrical power source, such as batteries, as well as an engine power source. The batteries are connected with the electrical drive units through an electronic control unit (ECU), which distributes the electrical power as required. The ECU also has connections with the engine and vehicle to determine operating characteristics, or operating demand, so that the electrical power units are operated properly as either a motor or a generator. When operating as a generator, the electrical power unit accepts power from either the vehicle or the engine and stores power in the battery, or provides that power to operate another electrical device or another electrical power unit on the vehicle or on the transmission.
There have been a number of electrically-variable transmissions proposed for vehicle operation. Examples of proposed electrically-variable transmissions are shown in U.S. Pat. No. 5,558,589 issued to Schmidt on Sep. 24, 1996, and assigned to the assignee of the present invention; U.S. Pat. No. 6,090,005 issued to Schmidt et al. on Jul. 8, 2000, and assigned to the assignee of the present invention; and U.S. Pat. No. 5,931,757 issued to Schmidt on Aug. 3, 1999, and assigned to the assignee of the present invention. The above-identified U.S. Pat. No. 5,931,757 defines the structure and operation of a variable two-mode, input-split, electro-mechanical transmission for a parallel hybrid electric propulsion system. U.S. Pat. No. 5,558,589 discloses a two-mode, compound-split, electro-mechanical vehicular transmission, and U.S. Pat. No. 5,558,595 issued to Schmidt et al. on Sep. 24, 1996, discloses a one-mode, input-split transmission. These and other patents describe various electrically-variable type transmissions.
One of the benefits of having an electrically-variable transmission incorporating more than one mode of operation is that each mode of operation will generally incorporate at least one mechanical point where one of the electrical power units is stationary, thereby reducing the electrical power input and providing a pure mechanical power flow path which is, of course, more efficient than a pure electrical power flow path.
Other hybrid type power transmissions are shown in U.S. Pat. No. 5,571,058 issued to Schmidt on Nov. 5, 1996; U.S. Pat. No. 5,577,973 issued to Schmidt on Nov. 26, 1996; U.S. Pat. No. 5,558,173 issued to Sherman on Sep. 24, 1996; and U.S. Pat. No. 5,558,175 issued to Sherman on Sep. 24, 1996, all of which are assigned to the assignee of the present invention.